The present invention relates in general to a serialization process for creating serial numbers of commercial products and in particular to a serialization process for a semiconductor wafer used in the manufacturing of thin film read/write heads of magnetic storage disk drives. The enhanced serialization process of the present invention utilizes a photolithography process in combination with a selective reactive ion etching process to rapidly create serial numbers of high optical contrast without causing undesirable deep trenches forming characters.
In a conventional magnetic storage system, a thin film magnetic head includes an inductive read/write transducer mounted on a slider. The magnetic head is mechanically coupled to a rotary actuator magnet and voice coil assembly by a miniature suspension and a gimbal assembly which is attached to an actuator arm positioned over a surface of a spinning magnetic disk. The slider design incorporates an air bearing surface to control the aerodynamic interaction between the magnetic head and the spinning magnetic disk thereunder. Air bearing surface (ABS) sliders used in disk drives typically have a leading edge surface and a trailing edge surface onto which thin film read/write heads are deposited.
In general, sliders are typically manufactured using a process involving a wafer made of a ceramic-like material such as A2O3TiC. The surface area of the wafer is generally divided into rectangular bars, which are usually referred to as slider bars that adjoin one another lengthwise. Each slider bar is further comprised of sliders adjoining one another along the length of the slider bar. In a conventional thin film head wafer process, the trailing edge surfaces of the sliders corresponds to the front side (or top side) of the wafer whereupon the read/write heads are further formed by various thin film processes. Thus, the backside of the wafer establishes the leading edge surfaces of the sliders.
In a conventional thin film head manufacturing sequence, read/write elements are placed onto the front side of the wafer in small rectangular areas that form the trailing edge surfaces of the sliders. These small rectangular areas are also referred to as dies. Upon forming the read/write elements, the wafer is diced into slider bars, each containing a row of sliders adjoining one another. The slider bars then undergo further processing. Initially, the ABS surfaces of the sliders are revealed by the dicing process, along the surface that represents the thickness of the wafer. Further processing is required to polish the ABS and to impart patterns that control the fly-height of the slider over the spinning magnetic disk. Thereafter, the sliders bars are sliced into individual sliders ready for the assembly sequence to form read/write heads-suspension assemblies (HSA).
As is common in mass production processes, a system of traceability usually must be established in order to provide a means for identifying the manufactured sliders. This identification process can be used for various process controls such as inventory control, quality assurance, etc. Such a traceability system usually exists in a form of permanent imprints of serial numbers created onto the leading edge surfaces of the manufactured sliders. Further, the characters of the serial numbers must be accurately read and recorded in an automated fashion. Hence, each character string must satisfy stringent Optical Character Recognition (OCR) rules and requirements. Thus, the serialization process forms an integral part of the manufacturing operation of the sliders at the wafer level.
One such conventional serialization process utilizes a laser scribing method. This process involves creating permanent imprints of serial numbers by a laser source onto the die areas of the ceramic substrate on the backside surface of the wafer. The laser source removes the substrate material by heat, known as ablation, to form characters constituting a serial number for each die. Because of heat generation, the laser scribing method suffers a number of disadvantages.
One of the significant problems with the laser scribing method is associated with potential wafer distortion due to heating by the laser energy, which may lead to damages to the slider and read/write devices. This distortion is caused by the thermal stresses induced onto the wafer surface as a result of the laser energy imparted on the backside of the wafer, thereby adversely affecting the desired flatness of the wafer. Furthermore, the flatness of the wafer is also affected by the surface stresses induced by the high density of characters forming on the die. The resulting distortion leads to subsequent manufacturing problems such as during photolithography, chemical mechanical polishing (CMP), etc. Additionally, if this serialization is performed after the fabrication of the read/write head element has been completed, such stresses could cause a degradation of the magnetic performance.
Another disadvantage associated with the laser scribing process is the deep trenches and loosely adherent re-melted material (slag) that form the characters acting as a reservoir for collecting debris generated during the ablation process an all subsequent processing, including slider-level processes. In addition, the loosely adherent slag represents a potential source of debris. In order to provide a sufficient optical contrast in the imprinted characters, the laser scribing process must create a sufficiently deep impression of the characters onto the backside of the wafer. The debris collected in this deep impression and the slag generated during the laser scribing process may therefore pose a reliability problem to the sliders and disk drive product as the debris and slag may loosen and become airborne and potentially interfere with the physical spacing between the flying slider and the spinning magnetic disk. The airborne particle can cause the magnetic data storage disk damage from the scratch during the disk rotation and potentially complete failure of the disk drive product.
Yet, another significant problem with the laser scribing method lies with the fact that this process is extremely slow, thus posing a significant manufacturing disadvantage. Because the laser scribing method is sequential, that is, characters are imprinted onto the die areas one at a time at an approximate rate of 12 characters per second, the serialization of each wafer may take up to 7 to 8 hours, since there are about 40000 dies per wafer with each die requiring about 8 characters.
A further disadvantage of the laser scribing method is the limitation on the character size and shape as dictated by the optics of the laser system and the dimensioning ability to meet the stringent OCR rules as character size decrease. Consequently, the serialization process using the laser scribing method is restricted to the backside of the wafer, since the characters formed by the laser scribing method would be too large to be accommodated by the limited surface area available of the front side of the wafer on which read/write elements are formed and too poor in character quality when attempting to scribe characters of requisite dimensions.
Another conventional serialization process uses a photolithography method to create characters on the front side of the wafer. While this method is advantageous to the laser scribing process in the ability to form smaller characters for the front side of the wafer, its disadvantages sufficiently offset its benefits. One such disadvantage is that the photolithography serialization method requires a process integration by adding more processing steps in depositions, photolithography, and precision etching in addition to the fabrication and use of expensive photomasks, thus resulting in an additional manufacturing complexity and hence increased manufacturing cost of the sliders.
Because of its ability to form smaller characters onto the front side of the wafer, the demand for more exacting methods and tooling is of a paramount importance for the photolithography serialization method. Hence, this results in a greater emphasis in high precision and expensive equipment to produce small-dimensioned characters with tight tolerances that would fit into a small surface area of the front side of the wafer, an area that shrinks disproportionately to reductions in slider size as in from Pico to Femto formats and beyond.
In light of the foregoing unresolved concerns with the conventional serialization techniques, there remains an unsatisfied need for an improved, fast serialization method. The serialization method should not require high precision, special expensive equipment, or special and expensive photomasks to produce small character sizes. Moreover, it should provide a means for enhancing the optical character contrast for sufficient OCR readability without the necessity for creating deep trenches onto the surface of the wafer.
In order to resolve the foregoing problems with the conventional serialization methods, it is a feature of the present invention to provide a new, enhanced serialization method. The enhanced serialization method utilizes a combined process involving photolithography with a unique selective reactive ion etching (RIE) process to produce a low cost, manufacturing efficient process for creating characters on the wafer at a high optical contrast level without causing deep trenches. In particular, the present invention realizes several advantages, among which are the following:
1. A low cost manufacturing process involving a non-rigid photomask generated by a laser plotter to create character specification for the wafer serialization pattern;
2. An alignment and exposure system to secure and retain the non-rigid photomask for creating the serialization pattern onto the wafer surface; and
3. A unique selective reactive ion etching (RIE) process that targets a certain chemistry of the substrate to produce a high optical contrast on the character imprints.
The new serialization process of the present invention offers several manufacturing advantages that overcome the deficiencies in the conventional serialization processes. The resulting manufacturing advantages translate into a significant reduction in wafer production time and cost, thereby leading to a shorter product cycle time and competitive pricing.